Glass-ceramic article and enamel suitable for the coating thereof

ABSTRACT

The present invention relates to a glass-ceramic article at least partly coated with at least one layer of an enamel formed from a glass frit having the following composition, the proportions being expressed as weight percentages: 
     
       
         
               
               
               
             
                   
               
                   
                 SiO 2   
                 50-66% and preferably 50 &lt; SiO 2  ≤ 65% 
               
                   
                 MgO 
                 3-8% and preferably 3-6% 
               
                   
                 Na 2 O 
                 7-15% 
               
                   
                 K 2 O 
                 ≤3% 
               
                   
                 Li 2 O 
                 ≤3%, in particular ≤2% 
               
                   
                 CaO 
                 ≤1% 
               
                   
                 BaO 
                 &gt;0-15% and preferably 5-15% 
               
                   
                 Al 2 O 3   
                 3-20% and preferably 3 &lt; Al 2 O 3  &lt; 20% 
               
                   
                 ZrO 2   
                 0-4%, preferably 0 &lt; ZrO 2  &lt; 4%, in particular 0.5-2% 
               
                   
                 ZnO 
                 &gt;0-5% 
               
                   
                 B 2 O 3   
                 &gt;0-6% and preferably 0 &lt; B 2 O 3  ≤ 5% 
               
                   
               
           
              
             
             
              
              
              
              
              
              
              
              
              
              
              
              
             
          
         
       
         
         
           
             the sum of the alkaline-earth metal oxides CaO+BaO moreover being between 8 and 15%, and the sum of the alkali metal oxides Na 2 O+K 2 O+Li 2 O moreover being between 7 and 20%. The invention also relates to the reinforced glass-ceramics obtained.

The present invention relates to an article (substrate, product) made ofglass-ceramic, in particular a glass-ceramic plate, intended, forexample, for covering or accommodating heating elements, such as forexample a hob, an oven door, or a chimney insert, or a fire screen,etc., to a process for obtaining said article, and to a novel enamelcomposition suitable for the coating thereof.

Sales of articles such as glass-ceramic hobs have been continuing togrow over the last few years This success is explained in particular bythe attractive appearance of such hobs and by the ease of cleaning them.

It will be recalled that a glass-ceramic is originally a glass, calledprecursor glass (or mother glass or green glass), the specific chemicalcomposition of which allows controlled crystallization to be induced bysuitable heat treatments, called ceramization. This partly crystallizedspecific structure gives the glass-ceramic unique properties.

At the present time, there are various types of glass-ceramic plate,each variant being the result of extensive research and many tests,given that it is very difficult to make modifications to these platesand/or to the process for obtaining them without risking an unfavorableeffect on the desired properties. In particular, to be able to be usedas a hob, a glass-ceramic plate must generally have a transmission inthe wavelengths of the visible range that is both low enough to mask atleast some of the subjacent heating elements when not in use and highenough so that, depending on the case (radiant heating, inductionheating, etc.), the user can, for the sake of safety, visually detectthe heating elements when they are in operation and/or can, whereappropriate, read the displays. It must also have a high transmission inthe wavelengths of the infrared range, especially in the case of hobswith radiant burners.

The glass-ceramic plates must also have a sufficient mechanical strengthas demanded in their field of use (for example, in accordance with theEN 60335-2-6 standard for hobs in the field of household electricalgoods). In particular, in order to be able to be used as hobs, theglass-ceramic plates must have sufficient resistance to the pressure andto the shocks that may arise (support and dropping of utensils, etc.).Generally, the glass-ceramic plates alone have a mechanical strengththat is expressed in particular by a scale factor (defined below)between 150 and 180 MPa.

Most current plates are of dark color, in particular black, but thereare also plates of lighter color (in particular white having, forexample, a haze of at least 50%, as described in patent FR 2 766 816),or even transparent plates provided with opacifying coatings. Amongknown (functional and/or decorative) coatings for glass-ceramic plates,there are conventionally enamels, based on glass frits and pigments, andcertain paints resistant to high temperature, based for example on alkydresins. In particular, enamels have the advantage of being able to bedeposited on the precursor glass (or mother glass or green glass) beforeceramization and of being able to be baked during the ceramization, andalso have the advantage of being able to withstand high temperatures(allowing the use of various heating means for the plate). However, theyhave the drawback of generally permitting only a single deposition (noenamel superposition is possible) and with a small thickness, otherwisethere is a risk in particular of the enamel flaking off and ofmechanically damaging the glass-ceramic plate. As regards paint, thismay be applied (if so required) as several layers. However, it must beapplied after ceramization (and therefore requires an additional bakingoperation) and remains limited to plates for induction burners(operating at lower temperature).

Glass-ceramic plates have also been proposed with coatings based onreflective layers deposited by magnetron sputtering or based on glassbatch materials incorporating special-effect pigments (aluminum oxide ormica flakes coated with metal oxides). However, the coatings based onlayers deposited by magnetron sputtering are more expensive since theyrequire a specific installation and are generally limited to plates forinduction burners, and their manufacture, carried out afterceramization, is more complex or tricky. As regards coatings based on aglass batch with special effect pigments, they have the same drawbacksas the abovementioned enamels.

More recently, an attempt has been made to develop novel enamels whichmake it possible to minimize the impact of the enamel on the mechanicalstrength of the baked enamel/glass-ceramic combination, for examplecompositions suitable for the most widespread dark glass-ceramic platescalled “arsenic-refined” (that is to say, obtained from a mother glasscomprising arsenic oxide at contents, for example, of around 0.5% to1.5% by weight), the compositions developed being in particular based ona glass frit formed of SiO₂ (present, for example, at 60.5% by weight ofthe frit), of MgO (present, for example, at 4% by weight), of Na₂O(present, for example, at 9.5% by weight), of Li₂O (for example at 5% byweight), of BaO (for example at 10% by weight), of ZrO₂ (for example 2%by weight), of ZnO (for example at 4% by weight) and of B₂O₃ (forexample at 5% by weight). However, these compositions are not suitablefor all the applications, these compositions being, for example, usedessentially as lower face of the hobs due to their low tear resistance.Furthermore, the novel formulations of glass-ceramic substratesdeveloped in order to make possible arsenic-free refining (with aparticular an arsenic oxide content of zero or less than 0.1%, it beingpossible for the arsenic to present problems in terms of health andsafety) have resulted in a modification to the interaction with thebaked enamel, embrittling much more the baked enamel/glass-ceramiccombination and requiring the development of other more suitablesolutions/formulations.

The object of the present invention was to provide improved novelglass-ceramic articles (such as plates), in particular to develop anenamel more suitable for the coating of different glass-ceramics, thisenamel embrittling the glass-ceramic as little as possible, inparticular for novel glass-ceramic substrates refined without arsenic,while having an improved delamination strength.

The present invention thus relates to a novel glass-ceramic article (orsubstrate), such as a plate, and a novel enamel for glass-ceramics, saidarticle being at least partly coated with at least one layer of saidenamel, this enamel comprising one (or being formed of one or from one)glass frit having the following (weight) composition, the proportionsbeing expressed as weight percentages (composition expressed as weightpercentages of oxides or else percentages by weight, based on theoxides, the constituents commonly being in this form in the enamelcompositions):

SiO₂ 50-66% and preferably 50 < SiO₂ ≤ 65% MgO 3-8% and preferably 3-6%Na₂O 7-15% K₂O ≤3% Li₂O ≤3%, in particular ≤2% CaO ≤1% BaO >0-15% andpreferably 5-15% Al₂O₃ 3-20% and preferably 3 < Al₂O₃ < 20% ZrO₂ 0-4%,preferably 0 < ZrO₂ < 4%, in particular 0.5-2% ZnO >0-5% B₂O₃ 0-6% andpreferably 0 < B₂O₃ ≤ 5%

the sum of the alkaline-earth metal oxides CaO+BaO moreover beingbetween 8 and 15%, and preferably between 8 and 12%, and the sum of thealkali metal oxides Na₂O+K₂O+Li₂O moreover being between 7 and 20%, inparticular between 7 and 15%.

Preferably, the glass-ceramic article according to the invention is aglass-ceramic plate, intended, for example, for covering oraccommodating at least one heating element, in particular intended to beused as a hob or as a wall (in particular a door or part of a door) ofan oven, or as a chimney insert, or else as a fire screen.

The present invention relates simultaneously to the (mineral) glasshaving the composition defined above, used for the frit and that makesit possible to produce the improved enamel and article according to theinvention, the enamel thus produced, having a composition that(initially) contains the particles (or frit) of said glass, and also inits form obtained by baking said composition, and the glass-ceramicarticle coated (usually over part or all of one face) with said enamel.

The present invention also relates to a process for manufacturing anarticle, in particular a plate according to the invention, in which theabove composition is applied, for example by screen printing or enameljet, to the precursor glass (or mother glass or green glass) articlebefore ceramization, said composition being baked during theceramization cycle, and/or in which the above composition is applied,for example by screen printing or enamel jet, to the glass-ceramicarticle after ceramization, then said composition is baked.

Advantageously, the glass-ceramic article, in particular theglass-ceramic plate, coated with the enamel according to the inventionhas a tensile strength at least of the same order as that of theconventional enameled plates in the case of the conventionalarsenic-refined glass-ceramics (that is to say with a (mother glass witha) composition comprising of around 0.5% to 1.5% by weight of arsenicoxide, or even with a composition comprising 0.2 to 1.5% by weight ofarsenic oxide), and a tensile strength with is significantly improvedcompared to that of the enamel plates in the case of the glass-ceramicsrefined without arsenic (that is to say, with a (mother glass with a)composition comprising less than 0.2% by weight, preferably less than0.1% by weight, in particular a content of less than or equal to 0.05%by weight, of arsenic oxide). The tensile strength is measured using aring-on-tripod bending test, on an enameled plate test specimen havingdimensions of around 70 mm×70 mm (the thickness of the plate moreovergenerally being around 4 mm), the enameled face being elongated. Thetest specimen rests on three 9.5-mm diameter balls each positioned atthe vertices of an equilateral triangle inscribed in a 40-mm diametercircle. A force is applied by pressing at the center (the load beingisotropic in this region) of the test specimen with a 10-mm diameterring. The rate of advance of the ring is around 5 mm/min. The resultsare interpreted using the Weibull model described in the followingarticle: “A Statistical Theory of the Strength of Materials”, RoyalSwedish Institute For Engineering Research, W. Weibull, Stockholm 1939,1-45. The data obtained revealing the average rupture stress is the dataknown as the “scale factor”, expressed in MPa (this scale factor being,in other words, the result of the processing, by the Weibull method, offlexural modulus of rupture (MOR) measurements).

Thus, the glass-ceramic article, in particular the glass-ceramic plate,coated with the enamel according to the invention advantageously (inparticular for the treated region) has a scale factor, obtainedaccording to the Weibull model following a bending test, of at least 80MPa, said factor possibly ranging up to 130 MPa at least, in the casewhere the glass-ceramic used is refined without arsenic, the scalefactor obtained for one and the same glass-ceramic coated this time witha conventional enamel generally being much lower (in particular notexceeding 60 MPa). It is thus observed that the glass-ceramics obtainedin the present invention are much less embrittled compared to thosetreated with conventional enamels.

Moreover, the glass-ceramic article coated with the enamel according tothe invention advantageously has an improved delamination strength, inparticular compared to the glass-ceramic articles coated with thereinforcing effect enamels recently developed, whether or not theglass-ceramic used is refined with arsenic. The delamination strength isgenerally measured by proceeding as follows: the enamel is deposited byscreen printing in a form of random patterns of millimetric size on aprecursor glass sheet and then baked during the ceramization or (isdeposited) on a glass-ceramic plate and then baked subsequently (afterthe ceramization). When the enamel is baked subsequently, the baking iscarried out at a temperature which allows the crystals to develop in theenamel, this temperature being chosen within the temperature range inwhich in particular a good coating and the formation of the crystals isobserved, this temperature range generally lying between 700 and 900° C.for the enamels according to the invention. Generally and preferably,this temperature is greater than approximately 250 to 300° C., comparedwith the dilatometric softening temperature of the enamel (or morespecifically of the glass/glass frit forming the enamel), and preferablycorresponds to (or is located just at or inside) the exothermic peak forcrystallization of the enamel.

After baking, two strips of adhesive tape (of “Scotch” type) with awidth of 12 mm (and the length of which depends on the size of thepatterns, it being possible for this length to range in particular up to30 cm), such as that sold under the reference Transparent Tape 550 by3M, are subsequently applied the one on the other to the enamel andremoved together with a sharp move perpendicular to the surface. Theglass-ceramic is subsequently observed under a microscope at a times 100magnification, the grade 0 being assigned where there is nodelamination, the grade 1 when there is less than one delamination permillimetric pattern, this delamination being small in size (less than25% of the size of the pattern) compared to the size of the pattern, thegrade 2 when there are from 1 to 3 delaminations per pattern, thesedelaminations being small in size (these combined delaminationsrepresenting less than 25% of the size of the pattern) compared to thesize of the pattern, the grade 3 when there are from 3 to 5delaminations per pattern (these combined delaminations representingless than 25% of the size of the pattern) compared to the size of thepattern, the grade 4 when there is on average 25% to 50% of the patterndelaminated, and the grade 5 when there is on average more than 50% ofthe pattern delaminated. The glass-ceramic article coated with theenamel according to the invention advantageously has a delaminationstrength grade of 0, corresponding to no delamination. The enamelaccording to the invention can thus advantageously be used on the upperface of the glass-ceramics.

The composition of the enamel according to the invention defined abovewill now be explained more precisely below. In this composition, theranges defined for each of the components are paramount for obtainingthe desired properties, respect for these ranges making it possible inparticular to simultaneously guarantee the production of the frit athigh temperature, good coating of the enamel on the substrate, thedesired mechanical strength and the chemical resistance, etc.

As indicated above, the composition mentioned preferably comprises morethan 3% of alumina Al₂O₃. Particularly preferably, it comprises morethan 4% of alumina (4<Al₂O₃<20%), in particular more than 5% of alumina(5<Al₂O₃<20%), or even at least 7% of alumina (7≤Al₂O₃<20%), inadvantageous embodiments according to the invention. Preferably again,it comprises more than 7% of alkali metal oxide Na₂O (7<Na₂O≤15%), inparticular at least 8% of Na₂O (8≤Na₂O≤15%). Preferably again, itcomprises less than 13% of alkaline-earth metal oxide BaO (0<BaO<13%, inparticular 5<BaO<13%); it especially comprises at most 12% of BaO(0<Bao≤12%, in particular 5<BaO≤12%). Preferably again, it comprises alow content of Li₂O, in particular of around 2% or less, or is devoid ofLi₂O.

It should be noted that, besides the constituents mentioned above, thecomposition may, if necessary, contain other constituents (for example,in the form of traces linked to the degree of purity of the rawmaterials) in a limited amount (less than 5%, generally less than 2%, inparticular less than 1%) as long as these constituents do not compromisethe desired properties, the composition also advantageously being freeof toxic metals such as lead, mercury, cadmium and hexavalent chromium.

It is surprisingly noted that the enamel according to the invention,based on the aforementioned glass frit, has an expansion coefficient(this coefficient being measured more accurately on the glass frit ofthe enamel, considered to be a glass) of at least 60×10⁻⁷ K⁻¹, inparticular at least 75×10⁻⁷ or even at least 80×10⁻⁷ K⁻¹, i.e. muchhigher than that of the glass-ceramic substrate. To date, it wascustomary to search for enamels having very low expansion coefficients,close to that of the glass-ceramic substrate, the behavior of the enamelon the substrate being assumed to be proportionally worse when thedifference between the expansion coefficients was high.

The enamel chosen according to the invention and the article, inparticular the plate, coated with this enamel have good thermalresistance that is compatible with the use of various types of heaters(induction, radiant, halogen, gas, etc. heaters), are scratch andabrasion resistant and resistant to heat shocks, have good agingresistance and offer, where appropriate (in particular when the frit iscombined with pigments and/or combined with another layer such as alayer of paint as explained below), an excellent compromise between theopacity commonly desired for enamels and the resistance to the variousmechanical stresses to which the coated plates are subjected, the enamelaffecting the mechanical strength of the glass-ceramics refined withoutarsenic on which the enamel is deposited less than conventional enamels,as desired according to the invention, while having a good delaminationstrength, as illustrated below.

From the process standpoint, the composition deposited does not differfrom a conventional enamel and is completely compatible with existingproduction lines, in particular, it may be applied by screen printingusing standard screen printing machines and fabrics. Compared with thinlayers deposited by magnetron sputtering, it is more economical and,being electrically insulating, it may be used, with no particularadjustment, with touch-sensitive controls, usually capacitivetouch-sensitive controls. It is also compatible with all types ofheating (in particular it withstands the high temperatures, of up to700° C., of the radiant heating elements, and is suitable for themagnetic fields of induction coils, etc.), unlike magnetron-sputteredpaints and, where appropriate, layers generally reserved for certaintypes of heating. It may also be deposited in any region of the plate(including the heater regions), in particular unlike paints.

Besides the glass frit (or glass particles) having the compositionexplained previously, the enamel according to the invention may alsocomprise other components. Remember that enamels are generally formed(before application to the substrate and baking) from a powdercomprising a glass frit (that has to form the glassy matrix) andpigments (as colorants in particular, these pigments possibly also beingpart of the frit), the frit and the pigments being based on metaloxides, and from a medium or “carrier” allowing the application and theprior adhesion of the enamel to a substrate.

The enamel according to the invention may thus comprise pigments, thecontent of pigment(s), added to the frit, in the assembly offrit(s)/pigment(s) of the enamel generally being between 5 and 40% byweight (relative to the assembly of frit(s)/pigment(s)), and preferablyranging from 10 to 35% by weight. The pigments for enamels may be chosenfrom compounds containing metal oxides such as chromium oxides, copperoxides, iron oxides, cobalt oxides, nickel oxides, zinc oxides,manganese oxides, cerium oxides, titanium oxides, or even based onalumina, etc. or may be chosen from copper chromates, cobalt chromates,etc. They are used as a function of the coloration and/or, whereappropriate, the opacity that it is desired to obtain. One example ofparticularly suitable pigments for adding to the frit according to theinvention is in particular a mixture of iron, chromium, copper, cobaltand nickel oxides, or a white pigment based on titanium oxide.

The glass frit and the pigments are conventionally in powder form beforebeing suspended in a medium. The particle size distribution of theassembly of frit(s)/pigment(s) in powder form is generally chosen sothat at least 90% by weight of the particles forming the powder have adiameter of less than 20 μm, advantageously less than 15 μm, inparticular less than 10 μm, notably less than 5 μm (i.e. so that (theparticles of) the frit(s)/pigment(s) assembly has or have a D90 lessthan 20 μm, advantageously less than 15 μm, in particular less than 10μm, notably less than 5 μm).

The frit of the composition according to the invention is conventionallyobtained by melting, at high temperature (more than 1000° C.) a mixtureof suitable (natural or synthetic) raw materials. The frit is thenmilled (generally in a solvent, such as ethanol, that is thenevaporated) in powder form, and if necessary pigments and/or opacifiersare added (before and/or after milling(s)). The pulverulent mixture(glass powder+pigments (and/or opacifiers)) obtained (and having, aftermilling(s) and/or other appropriate treatment(s), particles having onaverage diameter D90 less than 20 μm, advantageously less than 15 μm, inparticular less than 10 μm, notably less than 5 μm) is, afterevaporation if necessary of the milling solvent, subsequently suspendedin a medium in order to obtain a composition (paste) capable of beingdeposited onto a substrate.

The enamel composition according to the invention, in itsready-to-deposit form, thus generally also comprises a medium allowingadjustment to the viscosity desired for application to the substrate andenabling binding with the substrate. This medium, chosen in order toensure good suspension of the particles of frits and pigments and thatmust be consumed at the latest during the baking of the enamel, may beany medium or organic binder customarily used in the conventional enamelcompositions and may in particular comprise solvents, diluents, oilssuch as pine oil and other plant oils, resins such as acrylic resins,petroleum fractions, film-forming substances such as cellulosesubstances, etc. The proportion of medium in the ready-to-depositcomposition is preferably between 40 and 60% by weight of saidcomposition, preferably between 45 and 55% by weight.

The enamel composition before deposition onto an article, such as aplate, is therefore generally in the form of a stable liquid-solidmixture, of pasty consistency, with a viscosity suitable for thedeposition process (in particular by screen printing).

The layer of enamel deposited on the article or substrate, in particularthe plate, according to the invention generally covers at least one partof one face of the article (in particular of the plate), and may coverthe whole of said face (with the exception, where appropriate, ofregions and/or of resists, intended for example for the reading ofdisplays). The thickness of one layer of enamel after baking (whetherthe baking is carried out during the ceramization after deposition onthe precursor glass, or is carried out subsequently after deposition onthe glass-ceramic, as explained below) is from 1 to 10 μm, generallyfrom 2 to 3.5 μm, in particular from 2 to 3 μm, the thickness of theglass-ceramic being, for example, of around 3-4 mm in the case of aplate. In the latter case too, the layer defined according to theinvention may be deposited on the lower or upper face of the plate andis preferably deposited on the upper face.

The enamel according to the invention may be deposited as one orpossibly more layers and/or be combined, where appropriate, with otherlayers and/or act as an underlayer to another layer, such as a layer ofenamel (in particular of a different nature) or of paint, making itpossible in particular depending on the case to increase the thicknessesand/or to juxtapose two types of decoration (one of the layers formingfor example a base frame and the other forming a decoration or specificgraphics) and/or to procure a greater opacity, etc.

The enamel may, for example, be used where appropriate with at least onelayer of opacifying paint. The layer(s) of paint combined, whereappropriate, with the enamel according to the invention areadvantageously chosen so as to withstand high temperatures and to bestable with respect to their color and their cohesion with the plate,and so as not to affect the mechanical properties of the plate. Theyadvantageously have a decomposition temperature above 350° C., aregenerally based on one or more resins (such as a silicone resin, inparticular one modified by the incorporation of at least one alkydresin, or a polyimide, polyamide, polyfluorinated and/or polysiloxaneresin, such as the following resins: Dow Corning® 804, 805, 806, 808,840, 249, 409 HS and 418 HS, Rhodorsil® 6405 and 6406 from Rhodia,Triplus® from General Electric Silicone and SILRES® 604 from WackerChemie GmbH, etc.), and, where appropriate, they are filled (for examplewith one or more pigments or colorants) and optionally diluted so as toadjust their viscosity, the diluent being, where appropriate, removedduring their subsequent baking. The thickness of each paint layer may bebetween 1 and 100 microns (especially between 5 and 50 microns) and itmay be applied by any suitable technique, such as brush deposition,doctor blade deposition, spraying, electrostatic deposition, dipcoating, curtain coating, screen printing, etc. Generally, according tothe invention, it is deposited by screen printing, where appropriatefollowed by drying.

Advantageously, the substrate, in particular the glass-ceramic plate,coated with the enamel (obtained after baking) according to theinvention (the enamel if need be comprising pigments and/or beingcombined with a layer of paint, for example) has an opacity such that itmakes it possible in particular to mask underlying elements. The opacityis evaluated where appropriate by measuring (colorimetry in reflectioncarried out using a Byk-Gardner Color Guide 45/0 colorimeter) the colorvariation ΔE*, corresponding to the difference between the colormeasured on the face of the substrate opposite the face bearing theenamel, for the substrate placed on an opaque white background and thatfor the substrate placed on an opaque black background(ΔE*=((L_(B)*−L_(N)*)²+(a_(B)*−a_(N)*)²+(b_(B)*−b_(N)*)²)^(1/2)according to the equation established in 1976 by the CIE,L_(B)*,a_(B)*,b_(B)* being the colorimetric coordinates of the firstmeasurement on a white background and L_(N)*,a_(N)*,b_(N)* being thoseof the second measurement on a black background). Advantageously, theglass-ceramic substrate coated with the enamel according to theinvention has a ΔE* value less than or equal to 0.5, preferably lessthan or equal to 0.4.

As already mentioned, the present invention also relates to the processfor manufacturing articles, in particular plates, according to theinvention, in which the enamel composition according to the invention isapplied, preferably by screen printing, to the article of precursorglass (or mother glass or green glass) before ceramization, saidcomposition being baked during the ceramization cycle and/or in whichsaid composition is applied, preferably by screen printing, to theglass-ceramic article after ceramization, then said composition isbaked.

When the baking of the enamel is carried out subsequently (afterceramization, this procedure also being known as a process withrebaking), said baking can in particular be carried out advantageouslyat a temperature that makes it possible to develop crystals in theenamel (where appropriate while modifying the interface, thistemperature being chosen, for example, from the temperature range withinwhich good coverage with the enamel and the formation of crystals areobserved, this temperature range lying in particular between 700 and900° C. for the enamels according to the invention, this temperaturewhere appropriate being around 250° C. to 300° C. higher with respect tothe dilatometric softening temperature of the enamel (or more preciselyof the glass/of the glass frit forming the enamel), and preferablycorresponding to (or being located just at or inside) the exothermiccrystallization peak of the enamel. The enamel covering the substrateaccording to the invention is thus, where appropriate, crystallizedafter baking.

As a reminder, the manufacture of glass-ceramic plates generally takesplace as follows: the glass, having a composition chosen for forming theglass-ceramic, is melted in a melting furnace, the molten glass is thenrolled into a standard ribbon or sheet, by making the molten glass passbetween rolling rolls, and the glass ribbon is cut to the desireddimensions. The plates thus cut are then ceramized in a manner known perse, the ceramization consisting in firing the plates with the thermalprofile chosen to convert the glass into the polycrystalline materialcalled “glass-ceramic”, the expansion coefficient of which is zero oralmost zero and which is resistant to a heat shock possibly ranging upto 700° C. The ceramization generally comprises a step of progressivelyraising the temperature up to the nucleation range, generally located inproximity to the glass conversion range, a step of passing through thenucleation range over several minutes, a further progressive rise in thetemperature up to the ceramization hold temperature, the ceramizationhold temperature being maintained for several minutes, followed by rapidcooling down to room temperature. Where appropriate, the process alsoincludes a cutting operation (generally before ceramization), forexample using a water jet, mechanical scoring using a scoring wheel,etc., followed by a fashioning operation (grinding, beveling, etc.).

In the process according to the invention, the composition describedpreviously is deposited, either onto the glass precursor article or ontothe glass-ceramic article obtained after ceramization, generally in theform of a paste, the deposition of the layer of composition preferablybeing carried out by screen printing (the deposition nevertheless beingable to be carried out by another method if necessary), the thickness ofthe layer deposited (or wet film) being, for example, around a fewmicrons (in particular less than or equal to 20 μm, and generally lessthan or equal to 10 μm). After depositing the composition, the coatedarticle is generally dried (for example, via infrared heating or in anoven), generally at temperatures around 100-150° C., so as to evaporatethe solvent (medium), fix the coating and allow the article to behandled, which results in a dry coating, then depending on the case,undergoes a conventional high-temperature ceramization cycle (especiallyas mentioned previously), the baking of the layer accompanying theconversion of the substrate, or undergoes a (re)baking at a temperaturepreferably located in the crystallization zone as explained previously,the baking time being adapted as a function of the chosen temperature(for example, longer if the temperature chosen is lower), the coatingobtained then having a thickness generally around a few microns(generally between 1 and 10 μm, in particular from 2 to 3.5 μm). Theprocess with (re)baking is generally preferred as it makes it possibleto adapt the baking temperature in a more suitable manner as explainedpreviously and it makes it possible to obtain better mechanicalproperties for the glass-ceramic articles according to the invention.

In one embodiment, the article according to the invention is based on aglass-ceramic of black appearance, having a low light transmission ofless than 5% (such as the plates sold under the name Kerablack byEurokera) coated with the layer of enamel according to the invention,for example is based on glass-ceramics refined with arsenic having acomposition as described in patent application EP 0 437 228 or U.S. Pat.No. 5,070,045 or FR 2 657 079.

Preferably, the article according to the invention is based on aglass-ceramic having a content of arsenic oxide (expressed as As₂O₃) ofless than 0.2%, in particular of less than 0.1%, especially of less thanor equal to 500 ppm, or even zero, for example is based on glassceramics having a composition as described in patent application WO2012/156444 (these glass-ceramics being of black appearance, having alow light transmission, in particular of less than 5%, preferablybetween 0.8 and 2%, and being refined with tin).

The article according to the invention may also be of a light color,based on a transparent (such as the plates sold under the name KeraLiteby Eurokera and Keraglass) or a translucent glass-ceramic (such as theplates sold under the name Kerawhite, Kerabiscuit or Keravanilla byEurokera), coated with the layer of enamel according to the invention,said layer possibly being of decorative and/or functional use (forexample, possibly being intended for masking, at least partly, theunderlying elements when they are not in use, such as heating elementsand possible displays, while still allowing the heating elements andpossible displays to be detected when they are in use).

When the article according to the invention is a plate, said plate may,where appropriate, comprise reliefs and/or hollows and/or it may beprovided (or associated) with one or more additional functional ordecorative elements (frame, connector(s), cable(s), control element(s),display(s), for example what are called “7-segment” light-emitting diodedisplays or liquid crystal displays, electronic control panel withtouch-sensitive controls and digital display, etc.). The plate accordingto the invention may be mounted on an appliance, inside which the one ormore heating elements are placed, where appropriate without anintermediate complex with the aim of masking the interior of theapparatus from the user's view.

The invention also relates to the high-temperature-maintaining and/orcooking appliances (or devices) that include at least one substrate(plate or door) according to the invention (for example cookers,built-in cooktops, ovens, etc.). The invention covers both cookingappliances having a single plate and appliances having several plates,each of these plates having, where appropriate, a single heater ormultiple heaters. The term “heater” is understood to mean a cookinglocation. The invention also relates to hybrid cooking appliances, thehob(s) of which has (have) several types of heater. Furthermore, theinvention is not limited to the manufacture of hobs for cookers orcooktops. The plates manufactured according to the invention may, asdescribed above, also be other plates (chimney inserts, fire screens,etc.) that have to be very insensitive to temperature variations.

The examples which follow illustrate the results obtained with theglass-ceramic articles and enamels according to the invention (examples1 to 4) in comparison with reference examples relating to differentprevious glass-ceramic articles and enamels (reference examples 1 to 4).

In these examples, a glass-ceramic plate, one face of which is smooth(that which must receive the enamel layer in the form of millimetricrandom patterns) and the other face of which has evenly distributedspikes (with a height of 80 μm and an elliptical circumference of1.65/1.5 mm), is manufactured from a glass having a compositionindicated in each example.

This glass was melted at around 1600-1750° C. in an amount such that aglass ribbon was able to be rolled, from which ribbon glass plates withfinal dimensions of 56.5 cm×56.5 cm×0.4 cm were cut.

The plates were coated by screen printing on their upper face with acomposition, in the form of a screen-printable stable enamel (based on apowder having the composition specified in each example, the powderbeing made into a paste in a medium based on acrylic resin and on pineoil sold under the reference MX54 by Ferro for the purpose of depositingit onto the plate, and said medium being consumed at the latest duringthe baking of the enamel) using conventional polyester or polyamidefabrics, then dried at around 100-150° C.

The plates (green class or mother glass) coated with enamel were thenceramized on ceramic trays according to a cycle as described in patentapplication FR 2 657 079 (for glass-ceramics refined with arsenic) or inapplication WO 2012156444 (for glass-ceramics not refined witharsenic/glass-ceramics refined with tin).

Glass-ceramic plates coated with a layer of enamel were obtained, thethickness of the layer of enamel after baking being around 2.5 μm. Theseplates were cut to form 70 mm×70 mm test specimens, which were analyzedin terms of mechanical strength by measuring their scale factor(expressed in MPa) and also their Weibull modulus by means of aring-on-tripod bending test, the results being interpreted using theWeibull model, as described previously in the present text, thedecorated surface being in extension. The delamination strength of theenamel is also evaluated as described previously in the present text,using an adhesive tape sold under the reference Transparent Tape 550 by3M.

REFERENCE EXAMPLE 1

In this first reference example, the glass-ceramic plate wasmanufactured from a glass having a composition according to patentapplication FR 2 657 079 (conventional glass-ceramic refined witharsenic), this comprising, as weight percentages, the following oxides:

SiO₂ 69.44 Al₂O₃ 18.9 Li₂O 3.3 MgO 0.9 ZnO 1.55 BaO 0.75 K₂O 0.1 TiO₂2.6 ZrO₂ 1.75 As₂O₃ 0.51 Na₂O 0.2

The enamel used was a conventional enamel based on a powder comprising90% by weight of a glass frit having the following composition: SiO₂:48.6%; MgO: 3.8%; Na₂O: 2.6%; K₂O: 3.3%; Li₂O: 1.3%; CaO: 0.6%; BaO:17.8%; Al₂O₃: 7.1%; ZrO₂: 1.7%; ZnO: 8%; B₂O₃: 5.4% and 10% by weight ofa pigment based on iron, copper and chromium oxides. The scale factormeasured was around 88 MPa, the Weibull modulus (which reveals thedispersion of the results, the results being less dispersed as themodulus increases) being equal to 13.7. The delamination strength gradewas 0. The expansion coefficient of the glass frit/enamel (between 20and 400° C., this coefficient being calculated from the Appen modeldescribed in the work Glass—Nature, Structure and Properties, H.Scholze, Springer-Verlag, 1991) was 74×10⁻⁷ K⁻¹.

REFERENCE EXAMPLE 2

In this second reference example, the procedure was the same as inreference example 1, the enamel being replaced with an improved enamelhaving a reinforcing effect, this enamel being based on a powdercomprising 100% by weight of an alumina-free glass frit having thefollowing composition: SiO₂: 60.5%; MgO: 4%; Na₂O: 9.5%; Li₂O: 5%; BaO:10%; ZrO₂: 2%; ZnO: 4%; B₂O₃: 5%. The scale factor obtained was around180 MPa (with a standard deviation of 3.2 MPa), the Weibull modulusbeing equal to 27 MPa. The delamination strength grade was 3. Theexpansion coefficient of the glass frit/enamel was 99×10⁻⁷ K⁻¹.

Compared with reference example 1, a strengthening of the glass-ceramicrefined with arsenic was observed but a deterioration in thedelamination strength was also observed.

REFERENCE EXAMPLE 3

In this third reference example, the procedure was the same as inreference example 1, the glass-ceramic being replaced with aglass-ceramic manufactured from a glass not refined with arsenic havinga composition comprising, as weight percentages, the following oxides:

SiO₂ 64.8 Al₂O₃ 20.76 Li₂O 3.85 MgO 0.48 ZnO 1.5 BaO 2.5 K₂O 0.2 TiO₂3.0 ZrO₂ 1.3 CaO 0.5 As₂O₃ 0.05 Na₂O 0.6 SnO₂ 0.3 V₂O₅ 0.04 Cr₂O₃ 0.02Fe₂O₃ 0.1

The scale factor measured was of around 59 MPa, the Weibull modulusbeing equal to 21.2. The delamination strength grade was 0. Theexpansion coefficient of the glass frit/enamel was 74×10⁻⁷ K⁻¹.

A strong deterioration in the mechanical strength of the glass-ceramicnot refined with arsenic and coated with the conventional enamel wasobserved, the delamination strength remaining satisfactory.

REFERENCE EXAMPLE 4

In this fourth reference example, the procedure was the same as inreference example 3, the enamel being replaced with an enamel formed of90% of the frit of reference example 2 and of 10% of the pigment ofexample 1. The scale factor measured was of around 106 MPa, the Weibullmodulus being equal to 13.3. The delamination strength grade was 2. Theexpansion coefficient of the glass frit/enamel was 99×10⁻⁷ K⁻¹.

A deterioration in the mechanical strength of the glass-ceramic notrefined with arsenic and coated with this conventional enamel, comparedwith that obtained with a glass-ceramic refined with arsenic, wasobserved, the delamination strength remaining weak.

EXAMPLE 1

In this first example according to the invention, the procedure was thesame as in reference example 3, the frit of the enamel being replacedwith a frit according to the invention having the following composition:SiO₂: 54.5%; MgO: 4%; Na₂O: 9.5%; Li₂O: 2%; BaO: 10%; ZrO₂: 1%; ZnO: 4%;B₂O₃: 5%; Al₂O₃: 10%. The scale factor obtained was of around 85 MPa,the Weibull modulus being equal to 12.3. The delamination strength gradewas 0. The expansion coefficient of the glass frit/enamel was 84×10⁻⁷K⁻¹.

It was observed that the mechanical strength of the glass-ceramic notrefined with arsenic and coated with the enamel according to theinvention is at least maintained at the level of that normally obtainedwith the conventional glass-ceramics refined with arsenic, whereas itfalls much more strongly when the conventional enamels are used, thedelamination strength being at the same time improved compared with thatobtained using compositions which make it possible to retain a goodmechanical strength.

EXAMPLE 2

In this second example according to the invention, the procedure was thesame as in example 1, the frit of the enamel being replaced with thefrit according to the invention having the following composition: SiO₂:56.5%; MgO: 4%; Na₂O: 9.5%; Li₂O: 2%; BaO: 10%; ZrO₂: 2%; ZnO: 4%; B₂O₃:5%; Al₂O₃: 7%. The scale factor obtained was of around 85 MPa, theWeibull modulus being equal to 11.5. The delamination strength grade was0. The expansion coefficient of the glass frit/enamel was 85×10⁻⁷ K⁻¹.

It is here again observed that the mechanical strength of theglass-ceramic not refined with arsenic and coated with the enamelaccording to the invention is at least maintained at the level of thatnormally obtained with the conventional glass-ceramics refined witharsenic, whereas it falls much more strongly when the conventionalenamels are used, the delamination strength being at the same timeimproved compared with that obtained using compositions which make itpossible to retain a good mechanical strength.

EXAMPLE 3

In this third example according to the invention, the procedure was thesame as in example 1, the frit of the enamel being replaced with a fritaccording to the invention having the following composition: SiO₂:57.5%; MgO: 4%; Na₂O: 9.5%; Li₂O: 2%; BaO: 10%; ZrO₂: 1%; ZnO: 4%; B₂O₃:5%; Al₂O₃: 7%. The scale factor obtained was of around 93 MPa, theWeibull modulus being equal to 15. The delamination strength grade was0. The expansion coefficient of the glass frit/enamel was 85×10⁻⁷ K⁻¹.

It is here again observed that the mechanical strength of theglass-ceramic not refined with arsenic and coated with the enamelaccording to the invention is at least maintained at the level of that(or even greater than that, as in the present case) normally obtainedwith the conventional glass-ceramics refined with arsenic, whereas itfalls much more strongly when the conventional enamels are used, thedelamination strength being at the same time improved compared with thatobtained using compositions which make it possible to retain a goodmechanical strength.

EXAMPLE 4

In this fourth example according to the invention, the procedure was thesame as in example 2 but this time including an additional step ofmilling the frit/pigments assembly so as to achieve an average diameterD90 of the particles of the enamel of around 4.7 μm. The scale factor ofthe enameled plates obtained after application of the enamel to andbaking of the enamel on the glass-ceramic was of around 120 MPa, theWeibull modulus being equal to 9.7. The delamination strength gradewas 1. The expansion coefficient of the glass frit/enamel was 85×10⁻⁷K⁻¹.

An additional improvement is observed in the mechanical strength of theglass-ceramic not refined with arsenic obtained according to the presentexample compared with that observed above, the delamination strength atthe same time remaining improved compared with that obtained by usingother compositions which make it possible to obtain a good mechanicalstrength.

The plates according to the invention may especially be usedadvantageously to produce a new range of hobs for cookers or cooktops,or for producing elements of a wall or walls (for example doors) forovens, or for producing chimney inserts or fire screens, etc.

The invention claimed is:
 1. A glass-ceramic article at least partlycoated with at least one layer of an enamel formed from a glass frithaving the following composition: SiO₂ 50-66% by weight; MgO 3-8% byweight; Na₂O 7-15% by weight; K₂O ≤3% by weight; Li₂O ≤3% by weight; CaO≤1% by weight; BaO 7-15% by weight; Al₂O₃ >3-20% by weight; ZrO₂ 0-4% byweight; ZnO >0-5% by weight; and B₂O₃ >0-6% by weight,

wherein: a sum of alkaline-earth metal oxides CaO+BaO is between 8 and15% by weight; and a sum of alkali metal oxides Na₂O+K₂O+Li₂O is between7 and 20% by weight.
 2. The glass-ceramic article of claim 1, whereinthe glass frit comprises more than 4% by weight of alumina Al₂O₃.
 3. Theglass-ceramic article of claim 1, wherein the glass frit comprises atleast one of more than 7% by weight of Na₂O, and less than 13% by weightof BaO.
 4. The glass-ceramic article of claim 1, wherein the glass fritcomprises at most 2% by weight of Li₂O.
 5. The glass-ceramic article ofclaim 1, wherein the article is formed from a glass-ceramic comprisingless than 0.2% by weight of arsenic oxides.
 6. The glass-ceramic articleof claim 1, wherein the article is coated with the at least one layer ofenamel on at least one part of its upper face.
 7. A device for baking orfor maintaining a high temperature, the device comprising theglass-ceramic article of claim 1 and one or more heating elements. 8.The glass-ceramic article of claim 1, wherein the glass frit comprisesmore than 5% by weight of alumina Al₂O₃.
 9. The glass-ceramic article ofclaim 1, wherein the glass frit comprises more than 7% by weight ofalumina Al₂O₃.